In recent years, imaging systems such as video cameras and electronic still cameras have spread among the general public. On such imaging systems, a charge coupled device (CCD) type solid-state imaging device or a complementary metal oxide semiconductor (CMOS) type solid-state imaging device is mounted. In a solid-state imaging device, a plurality of pixels are arranged in a two-dimensional matrix, signal charges generated by a photoelectric conversion unit such as a photodiode provided in the pixel on which light is incident are held in a charge holding unit, amplified by an amplifying unit provided in the pixel, and output as a pixel signal. In this case, in a general CMOS type solid-state imaging device, pixel signals from pixels arranged in a two-dimensional matrix are sequentially read for each row.
However, in an imaging system, there is an operation in which processing for a pixel signal of resolution corresponding to the total number of pixels included in a solid-state imaging device (full resolution) is not necessary. For example, while the processing for a pixel signal of full resolution of the solid-state imaging device is performed when a still image is captured in an imaging system, it is not necessary to perform the processing on a pixel signal of full resolution of the solid-state imaging device when a moving image is captured or when an image for confirming a subject, that is, a live view image (through image), is displayed on a display device provided in the imaging system. Further, in control such as auto exposure (AE), auto white balance (AWB), or auto focus (AF) performed when a still image is captured, or in confirmation of a dynamic range, confirmation of color of an entire image and, confirmation in image processing when a still image is generated such as confirmation of flickering of a fluorescent lamp, it is not necessary to perform the processing on a pixel signal of full resolution of the solid-state imaging device, and pixel signals of only some of the pixels may be read and the processing may be performed on the pixel signals.
This is because resolution necessary for each process is low in capturing of a moving image, generation of a live view image, and control when the still image is captured, in comparison with a case in which the still image is captured. Rather, in the capturing of a moving image, the generation of a live view image, or the control when the still image is captured, a frequency of update of an image to be processed is required to be high, that is, a frame rate is required to be high, rather than the resolution of the image. Further, in AF, AE, or AWB detection, an even higher frame rate is required.
Therefore, as a technology for reading pixel signals from a solid-state imaging device at a high frame rate, there is a method of decimating and reading pixel signals and adding and averaging signals of a plurality of pixels. This method has already been realized in a large number of imaging systems. A problem with such a method of reducing resolution and reading pixel signals is that, since a photoelectric conversion unit and a charge holding unit are occupied when the pixel signals are read, it is difficult to periodically overlay reading of pixel signals of full resolution of the solid-state imaging device and reading of pixel signals at decreased resolution. For example, in a digital camera, after a shutter is pressed at the time of capturing a still image at full resolution, it is difficult to perform photographing at a higher frame rate as described above while the signals are being read from the solid-state imaging device. As a method for solving this, for example, a technology as shown in Japanese Patent No. 5226552 is disclosed. In the technology disclosed in Japanese Patent No. 5226552, reading of pixel signals from a solid-state imaging device is decimated for each row of pixels to increase a frame rate. For example, the pixel signals of pixels included in the solid-state imaging device are first read to increase the frame rate through decimation at a rate of one line to seven lines, and then pixel signals of a row that is not decimated are read. Accordingly in the technology disclosed in Japanese Patent No. 5226552, pixel signals for generating a moving image or a live view image, and pixel signals for generating a still image can be acquired.
Further, for example, a technology of a solid-state imaging device including pixels including a plurality of charge accumulation circuits that accumulate signal charges generated by photoelectric conversion units is also disclosed as in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2013-030913. In the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2013-030913, respective signal charges generated by photoelectric conversion units in each exposure are accumulated in respective charge accumulation circuits. Accordingly, for example, solid pattern noise or the like is reduced, and output linearity of autofocus is improved.